Organic EL devices, being self-light emitting devices, are brighter and more visible than liquid crystal devices, enabling a clear display. Accordingly, much research has been conducted on organic EL devices.
In 1987, C. W. Tang et al. of Eastman Kodak Company successfully developed a practical organic EL device by creating a layered structure that divides various roles for light emission among different materials. This organic EL device is configured by laminating a layer of a fluorescent body capable of transporting electrons and a layer of an organic substance capable of transporting holes. By injecting positive charges and negative charges into a layer of the fluorescent body and causing light to be emitted, a high luminance of 1,000 cd/m2 or more at a voltage of not more than 10 V can be obtained.
Many improvements for the practical utilization of organic EL devices have been made to date. For example, it is commonly known that a high efficiency and durability can be achieved by dividing up even further the various roles of the layered structure and creating a layered structure that has, provided on a substrate, an anode, a hole injection layer, a hole transport layer, a luminous layer, an electron transport layer, an electron injection layer and a cathode.
To further improve the luminous efficiency, efforts are being made to utilize triplet excitons and the use of phosphorescent light-emitting compounds is being investigated.
In addition, devices that utilize light emission by thermally activated delayed fluorescence (TADF) have also been developed. For example, in 2011, Adachi et al. at Kyushu University achieved an external quantum efficiency of 5.3% with a device that uses a thermally activated delayed fluorescent material.
The luminous layer is fabricated by doping a charge transporting compound that is generally called the host material with a fluorescent compound or phosphorescent light-emitting compound or with a material radiating delayed fluorescence. Selection of the organic materials in an organic EL device has a large influence on device characteristics such as efficiency and durability.
In an organic EL device, the charges injected from both electrodes recombine in the luminous layer, resulting in light emission. How efficiently the hole and electron charges are delivered to the luminous layer is important, and the device to have an excellent carrier balance is required. Also, increasing the hole-injecting properties and increasing the electron-blocking properties that block electrons injected from the cathode improves the probability of holes and electrons recombining, and confining excitons generated within the luminous layer enables a high luminous efficiency to be obtained. Because of the important role thus played by the hole-transporting material, there exists a desire for a hole-transporting material which has high hole-injecting properties, a high hole mobility, high electron-blocking properties and moreover a high durability to electrons.
With respect to the device life, the heat resistance and amorphousness of the material are also important. In a material having a low heat resistance, thermal decomposition arises even at low temperatures due to the heat generated during device operation, resulting in degradation of the material. In a material having low amorphousness, crystallization of the thin film occurs in a short time, leading to device deterioration. Hence, the material to be used is required to have high heat resistance and good amorphousness.
N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (NPD) and various aromatic amine derivatives are known as hole-transporting materials that have hitherto been used in organic EL devices (see, for example, PTL 1 and PTL 2). Although NPD has a good hole-transporting ability, the glass transition temperature (Tg), which serves as an indicator of heat resistance, is low at 96° C. Hence, under high-temperature conditions, the device characteristics deteriorate due to crystallization.
Among the aromatic amine derivatives mentioned in PTL 1 and PTL 2 are also compounds having an excellent hole mobility of at least 10−3 cm2/Vs, but because the electron-blocking properties are inadequate, some electrons end up passing through the luminous layer, making it unlikely that, for example, an enhanced luminous efficiency is achieved. To further increase efficiency, there has existed a desire for a material which has higher electron-blocking properties and forms a thin film that is more stable and has a higher heat resistance.
Aromatic amine derivatives with high durability have also been reported (see, for example, PTL 3). However, these have been used as charge-transporting materials for electrophotographic photoreceptors, there are no examples of their use in organic EL devices.
Arylamine compounds having a substituted carbazole structure have been proposed as compounds having improved characteristics such as heat resistance and hole-injecting properties (see, for example, PTL 4 to PTL 6). However, in devices which use these compounds in the hole injection layer or hole transport layer, although the heat resistance and luminous efficiency have been improved, the results are still insufficient. An even lower driving voltage and an even higher luminous efficiency are desired.
In order to improve the device characteristics of organic EL devices and enhance the yield in device fabrication, there exists a desire for a device which, by combining materials that have excellent hole and electron injecting and transporting performances and form thin films of excellent stability and durability, enables holes and electrons to recombine at a high efficiency and has a high luminous efficiency, a low driving voltage and a long life.
In addition, to improve the device characteristics of organic EL devices, there exists a desire for a device which, by combining materials that have excellent hole and electron injecting and transporting performances and form thin films of excellent stability and durability, are balanced in careers and achieves a high efficiency, a low driving voltage and a long life.